1. Field of the Invention
The present invention relates to the detection of moving targets in the presence of fixed targets by means of a radar sending out coherent pulses all of which, in the course of one burst, have the same initial phase shift with respect to their modulation carrier.
2. Description of the Prior Art
In response to an illumination by a burst of coherent pulses, a target returns the burst of pulses as an echo, and preserves or does not preserve their coherence depending on whether it is a fixed target or a moving target. This is because the Doppler effect due to the radial shifting of a target in relation to the pulse source prompts the appearance of a phase shift from pulse to pulse. Thus, the signal received in return, in response to the transmission of each coherent pulse of a burst, which results from the superimposition of responses from fixed and moving targets, has a component that is invariant from pulse to pulse and is due to the fixed targets and a component that fluctuates in phase from one pulse to another and is due to the moving targets.
For the revealing of the moving targets, the invariant component of the reception signal received as an echo of an interrogation pulse due to the illuminated fixed targets is a source of inconvenience. This is all the more so as its level is often higher than that of the variable component of the reception signal due to the moving targets. It is usual to attempt to eliminate it or at least to weaken it before making use of the reception signal, by exploiting the fact that it is found in an identical state in all the echoes in response to the pulses of a burst while the component due to the moving targets fluctuates in phase from one echo to another. In one standard method, before exploiting the received signal and processing it to separate the targets in terms of distance and possibly speed, a subtraction is made, from the signal received in response to the transmission of a pulse, of the signal received in response to a previously transmitted pulse. This operation, which enables the elimination of the fixed echoes, is generally done by means of a delay line, which delays the reception signal by one pulse repetition period, and a subtractor. The delay line may be an analog line. It then works in intermediate frequency band. It may also be digital and work in video frequency band.
The presence of a high-level parasitic component in the reception signal due to the echoes from fixed targets raises problems of saturation of the demodulator which gives the video signal and of the analog-digital converters used to enable digital processing with a view to the detection of the targets. It is therefore advantageous to eliminate or at least reduce the echoes from the fixed targets in the intermediate frequency.
One method used to eliminate the echoes from fixed targets in the intermediate frequency, described in the French patent application No. FR-A-2.687.479, consists in generating an intermediate frequency replica of the signal received in response to the first pulse of a burst. This is done by the sampling, after demodulation, of the amplitude of this signal measured according to a logarithmic scale and of its phase and by the synthesizing of the replica by amplitude and phase modulation of the intermediate frequency carrier by means of the samples obtained and the subtraction, in intermediate frequency band, of this replica from each of the signals received in response to the subsequent pulses of the burst.
This method of elimination of the echoes of fixed targets has the advantage of not using an analog delay line with a period equal to the pulse repetition period. Such a delay line is difficult to make and its insertion gives rise to phenomena of attenuation and distortion that are difficult to master. However, it may happen, in the presence of strong echoes from fixed targets, that there may be a certain degree of saturation giving rise to the formation of a fairly approximate replica at the first pulse of the burst.
The present invention is aimed at overcoming this drawback by forming the replica in response to the first pulse of a burst by making successive approximations by means of an iterative process based on a first sampling relating to the response to the first pulse and then a second sampling relating to the error that persists between the response to the first pulse and a temporary replica synchronized from the first pulse, and to a combination of the two samplings to prepare a more precise replica of the response to the first pulse.